Plasma-chemical reactor

ABSTRACT

The reactor contains the reaction chamber ( 1 ), the means for the removal of aim product from the reaction chamber ( 7 ), at least a pair of electrodes in the reaction chamber, each made as an open container ( 2 ) filled with metal ( 3 ), placed in such a way that voltage applied between them strikes an arc discharge in the space between electrodes, and the means for supplying the reaction chamber with plasma forming gas which feeds the reaction chamber with plasma forming gas to form the vortex flow of the gas in the space between the electrodes.

FIELD OF ART

The invention relates to chemical reactors employing electric dischargeplasma as the source of high temperature. This reactor can be used inchemical engineering, metallurgy and other industries for the productionof such chemical products as gas synthesis, hydrocarbons etc., as wellas in environment control, health protection, biosphere preservation asan effective device for the decomposition of depleted persistent toxicagents and their removal from industrial exhaust and waste.

STATE OF THE ART

Plasma chemical reactors are well known. Their main element is adischarge chamber provided with two electrodes and with an inlet andoutlet for plasma forming gas. Thus there are plasma chemical reactorsemploying some inert gas such as nitrogen, argon or hydrogen as a plasmaforming gas. Plasma is formed in the reactor by exciting the is plasmaforming gas with an electric discharge in a specially equipped chamberand then fed to the separate reaction chamber with the reaction mixturewhere the plasma induces chemical reactions yielding the desired targetproduct.

Among such reactors we can name, for instances, the device forhigh-temperature chemical reactions yielding powders of high-puritymetals of IVb, Vb, VIb groups of the periodical table, i.e. titanium,tungsten, molybdenum etc., or their alloys as well as for thehalogenation of metal oxides and the synthesis of hydrocarbons such asacetylene, gasoline etc. The device is equipped with a plasma generatorproducing plasma by means of an electrical discharge between the anodeand cathode when plasma forming gas such as argon or nitrogen is fedthere. The plasma from the generator and gaseous reaction mixture arefed to a special reaction zone in the device below the anode. A chemicalreaction proceeding afterwards in the reaction zone yields the desiredproduct. After the reaction the flow of the reaction mixture containingthe desired product is quenched in the quenching zone and separated intoseveral flows later combined in a collector zone where the pure targetproduct is extracted (U.S. Pat. No. 3,840,750).

One of the well-known devices is also a plasma reactor for the thermalcracking of substances, mainly hydrocarbons. The plasma is generated ina discharge chamber specially equipped with axially fixed anode andcathode between which an electrical arc is formed, through which aplasma forming gas—hydrogen or nitrogen—is passed. The discharge chamberis connected with a mixing chamber where all the necessary reagentsforming the initial hydrocarbon reaction mixture of preset compositionare fed. Then the initial reaction mixture heated to several thousandsdegrees Celsius is introduced directly into the reaction chamber wherethe desired product is formed. The target product is isolated viasubsequent rapid cooling of the reaction mixture by cold quenching gasin the free space over the reaction chamber. Then the final product isfed to the scrubber to wash the gas off (U.S. Pat. No. 3,622,493). Theabove-described plasma chemical reactors are bulky, have cornplicatcddesign and are expensive. In addition the service life of electrodes inthe discharge chamber is rather short because of the erosion caused byhigh voltage, strong currents and plasma particle bombardment of thesurface which necessitates their frequent replacing requiring too manyshutdowns of the reactors.

Some plasma chemical reactors are known where the reaction gas servesdirectly as the plasma forming gas. Their design is much simpler thanthat of those described above because they have a single reactionchamber equipped with a pair of electrodes through which the reactiongas is passed between them under the application of high voltagestriking an electric arc discharge. Among the plasma chemical reactorsof the similar design are, for instance, the reactor whose reactionchamber is equipped with an anode and cathode to which high voltage isapplied, as well as with inlet and outlet devices for reagent inlet andthe outlet of the target product (U.S. Pat. No. 3658673). The reactionmixture is passed between the electrodes and set into forward-rotawymotion leading to the formation of vortex stabilizing the plasma arcbetween electrodes. In this reactor the electrodes in addition to theabove-mentioned factors are exposed to the aggressive chemical medium,their surface is soon eroded and unusable and are to be replacedfrequently (every several hours). The electrodes erosion increases withthe increasing in the electrical discharge current thus limiting themaximum current value and imposing restriction on the maximumproductivity of plasma chemical reactor.

There is also a plasma chemical reactor for the thermal decomposition ofchemical industrial waste. The reactor consists of the reaction chamberwith two electrodes between which the treated gas mixed with the oxygenat the voltage of 100-3000 V causing current of 50-1000 A (U.S. Pat. No.5,206,879). This reactor, as well as above-mentioned ones, requiresfrequent (every several hours) electrodes replacement, because thepresence of oxygen significantly accelerates the erosion of theelectrodes. All this also imposes constraints on its productivity.

DISCLOSURE OF THE INVENTION

The object of the present invention is the sufficient prolongation ofthe plasma chemical reactor electrodes service life with correspondingdecreasing of the expenses of its exploitation and increasing of thereactor productivity.

This is attained by the suggested design of a plasma chemical reactorconsisting of reaction chamber, means for supplying the plasma forminggas and the means for removal gf the aim product, at least a pair ofelectrodes placed in the reaction chamber, so as to provide a gas arcformation in the electrode gap on the application of the electricvoltage. Each electrode is shaped as an open container filled with metaland the means for supplying the reaction chamber with plasma forming gasto be fed to the electrode gap as a vortex.

The electrodes in the reaction chamber should be arranged horizontally,because the electrical arc initially emerging between solid electrodesgradually melts them and in the course of long-time operation theelectrodes become liquid. For example, each container can be made as atank at the bottom of the chamber, with the setting from the refractorymaterial with dielectric properties, i.e. ladle brick. The container isfilled with metal pieces, particles or ordinary scrap metal. To maintainthe arc discharge at voltage supply to the electrodes, the ratio of suchtechnological parameters as voltage value, the distance betweenelectrodes, the consumption of plasma forming gas fed to the chamber,its composition, etc, specified for each construction separately, isimportant.

The melting of the metal in the containers because of the electrical arcleads to the liquefaction of the electrodes working surfaces (thesurfaces, between which an electrical arc burns) during the reactoroperation, so the electrode is not subject to erosion in the broadsense, but rather to the slow evaporation of the metal and the decreasein its mass. Due to the big mass of the electrode, the reactor canoperate for a long time without stops depending on the volume ofelectrode container and the velocity of the metal evaporation.

To additionally increase the period of continuous operation of thereactor, it is necessary to provide every container with a metal supplymeans. Those can have the shape of chutes going to containers, alongwhich metal pieces are fed.

The voltage is supplied to the electrodes directly through the metalfilling the containers. For this purpose every container is providedwith special means for voltage supply to the filling metal, which may bedesigned as a channel with a metal conductor inside, one end of which isconnected with the container, melts together with the filling metal, andthe other end, connected with the contacts for the electrical voltagesupply, remains solid. Hearth water-cooled electrode may also be usedfor the same purpose.

The means for the supply of plasma forming gas to the reaction chamberbetween electrodes can have different constructions. It is important forthe means to form a vortex gas flow stabilizing the electric arc andprecluding the contact between the electrical arc and chamber parts. Oneof acceptable and simple designs of this means includes the wall made ofrefractory dielectric material. Its size can be different, but heightand width must prevent the arc discharge established at voltage supplyto electrodes from bending around this wall. The wall may be made as apartition between the bottom and ceiling of the reaction chamber,dividing its volume in two parts, if it is technically feasible. Thereis a discharge channel directing the arc discharge in the wall. It maybe made as a central through cylindrical hole or as a tube fixed in sucha hole and bent in such a way that its ends were directed towards thesurface of electrodes. Gas channels connected with the discharge channeland a plasma forming gas source through which the plasma forming gas isinjected directly to inter-electrode space. The electric arc is formedin the discharge channel on the wall and enters the reaction chambervolume between electrodes on the both sides of this wall.

In order to stabilize the arc discharge, the plasma forming gas is setinto rotation with vortex formation. The resulting vortex must form acooler and therefore denser plasma forming gas layer between theelectric arc and the discharge channel wall which insulates the chamberwalls and its other parts. For this purpose gas channels are placed atan angle to the surface of the discharge channel they are connectedwith, i.e. the gas is introduced tangentially. The plasma forming gasenters the discharge channel at an angle to its wall and then forms avortex.

If the discharge channel is not sufficient for the vortex formation, themeans for the supply of plasma forming gas to the reaction chamber canbe additionally equipped with a vortex chamber of a well-known design.For example, it can be shaped as a cylindrical groove inside the channelwall of means reaction gas supplying, which is connected with thedischarge channel and has bigger diameter than that of the channel, andthe gas channels are brought out to its walls at an angle. Thesuccessive arrangement of several vortex chambers in the dischargechannel is also possible. In this case plasma forming gas passes throughevery chamber and it can be of one composition and fed through all thechannels from the same source or it composition may differ and multiplesources of plasma forming gas can be used.

The walls of discharge channel can be cooled by water to prevent theirdestruction by high temperatures.

The reaction chamber volume must be much larger than the above-mentioneddevice to serving as the plasma source for sustaining plasma chemicalreactions. The plasma forming gas is heated to high temperature in thedischarge channel and is passed into the reaction chamber volume whereit is combined with other reagents or the reaction mixture and inducesthe further chemical reaction.

There are different ways of conducting chemical reactions in thereactor. Thus, plasma forming gas feeds through a special means can bean inert gas such as argon, nitrogen, etc., or reaction mixturespecially prepared outside the chamber, or individual reagents involvedin the aim reaction. When a plasma forming inert gas or individualreagents are used the reaction chamber must be equipped with specialconduits for dispensing the prescribed quantities of the reactionmixture made outside the chamber or all the necessary reagents (liquid,solid, gaseous). If the plasma forming gas is reaction mixture,additional device for supplying the specific gas or individual reactantscan be provided, if necessary.

For the aim product output from the reaction chamber it is equipped withthe means for its removal.

To fire an electrical arc and to start the operation of the reactor thewell-known methods are used:

-   -   1. Electrodes are linked through the discharge channel by a wire        connector. Plasma forming gas is passed to the chamber through        gas channels, and electrical current is applied to the        electrodes. The wire connector instantly becomes hot and        explodes generating a plasma channel striking an arc discharge.    -   2. Metal plates—electrode links—are installed into the discharge        channel walls. The plasma forming gas is fed through the gas        channels and simultaneously the voltage is applied to the        electrodes. Further, high-voltage pulse is sent to the electrode        links, resulting in the plasma forming gas break-down, which in        its turn initiates the formation of arc discharge.

The required reactions take place in the reaction chamber under hightemperatures of arc discharge and electric arc effects the melting ofelectrode metal filling the containers, which are eventually filled withmelted metal.

Since waste accumulation in electrode containers is possible in thecourse of the reactor operation, waste removal devices can prove useful,e.g. every container could be equipped with a siphon.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the scheme of the reaction chamber of the plasma chemicalreactor where:

-   (1)—reaction chamber, (2)—electrode container, (3)—metal filling the    electrode container, (4)—chute for providing electrode container    with metal, (5)—a wall in the electrode gap, (6)—discharge channel,    (7)—aim product outlet, (8)—gas channels for plasma forming gas,    (9)—vortex chamber, (10)—additional inlets for reaction gas and    reagents, (11)—device for voltage supply to electrode container,    (12)—device for slag removal.

BEST MODES CARRYING-OUT THE INVENTION

By way of example of plasma chemical reaction operation synthesis-gasproduction process can be considered. The reactor contains a largevolume reaction chamber (1) with two refractory lining tanks (2), filledwith scrap iron (3), at the bottom. Each tank has a special channel witha connector for electrical voltage supply directly to the scrap metal inthe tank (11). Vertical wall of the refractory brick (5), fixed at thebottom of the chamber (the wall is not connected with the ceiling of thechamber) is placed between the tanks. This wall has such a height andwidth that arc discharge is unable to bend around it when the voltage isapplied to the electrodes. There is a tube-like discharge channel (6),bent in such a way that its ends (and outlets accordingly) were directedto the surface of the electrodes in the wall. The vortex chamber isshaped as an additional groove in the discharge channel with thediameter lager than that of the channel itself. There are also gaschannels (8) for plasma forming gas, one end of which is connected withthe source of this gas, and the other end tangentially entering thevortex chamber (9). The length of the discharge channel must besufficient for the plasma forming gas entering through gas channels toform a vortex, able to stabilize the electric arc. The reactor isstarted up by the initiation of electrical breakdown by a high-voltageelectrical pulse. Further, reagents—hydrocarbons and water steam—are fedto the chamber volume through a special conduit in the ceiling of thechamber (10). Plasma is a source of high temperature necessary forchemical reaction to proceed. Small quantities of iron vapors comingfrom the molten electrode surface are the catalyst of the reactionyielding synthesis as increasing the reaction rate. The target producti.e. the synthesis gas is withdrawn from the reaction chamber throughtwo outlets in the opposite walls (7). Coal, natural gas or otherhydrocarbons, instead of vapor, or oxygen-vapor mixture can be put intothe reaction chamber. Inert gas, water steam, natural gas or othersuitable substances can be used as plasma forming gas. To additionallyincrease the productivity of the plasma chemical reactor it is possibleto increase the volume of its reaction chamber. The chutes (4) serve tofill the electrode containers with metal. There are siphons (12) forslag removal from the electrode surface.

When used for other chemical processes the reactor can be customized andfurther developed.

In the present plasma chemical reactor the service life of theelectrodes is fairly long because they have big mass and their surfaceduring the reactor operation is in molten state. The liquid surface ofthe electrodes is not prone to erosion, so no shutdown of the reactorfor the electrode replacement will be necessary.

INDUSTRIAL APPLICABILITY

Plasma chemical reaction may be applied in metallurgy, chemical andother industries for the production of chemical products and inenvironment protection for treating noxious production waste.

1. A plasma chemical reactor comprising: a reaction chamber; gas supplymeans for supplying the reaction chamber with plasma forming gas; meansfor the removal of the aim product from the reaction chamber; and atleast a pair of electrodes in the reaction chamber, each electrode beinglocated in an open container filled with metal, and disposed in such away that voltage applied between them strikes an arc discharge in thespace between the electrodes; and wherein the gas supply means feeds theplasma forming gas to the reaction chamber between the electrodes toform a vortex flow of the gas in the space between the electrodes. 2.The plasma chemical reactor according to claim 1, wherein the arcdischarge in the space between the electrodes is such that it inducesmelting of the metal within the open containers.
 3. The plasma chemicalreactor according to claim 1, wherein the electrodes in the reactionchamber are arranged horizontally.
 4. The plasma chemical reactoraccording to claim 1, wherein the reaction chamber includes additionalinlets for individual reagents and reaction mixture.
 5. The plasmachemical reactor according to claim 1, wherein the electrode containersand the gas supply means are formed of a heat-resistant dielectricmaterial.
 6. The plasma chemical reactor according to claim 1 furtherincluding a chute communicating with each electrode container, thechutes being constructed and arranged for filling the associatedcontainer with metal.
 7. The plasma chemical reactor according to claim1, further including voltage supply means for applying voltage to metalwithin each electrode container, the voltage supply means each beingcomprised of a channel within which an electrical conductor is received,the conductor having a first end which is brought in contact with themetal filling the container and which melts together with the metal, andthe second end which is connected to an external voltage source, andwhich remains solid during operation.
 8. The plasma chemical reactoraccording to claim 1, wherein the gas supplymeans is comprised of: avertical wall at the bottom of the chamber; a discharge channel; aplurality of internal gas channels, the gas channels having respectiveoutlet ends communicating with the discharge channel and respectiveinlet ends communicating with at least one source of plasma forming gas;the gas channels being located at an angle to the wall of the dischargechannel such that plasma forming gas forms the vortex in the dischargechannel.
 9. The plasma chemical reactor according to claim 8, whereinthe discharge channel is comprised of a cylindrical hole through thewall of the gas supply means.
 10. The plasma chemical reactor accordingto claim 8, wherein the discharge channel is comprised of a tubularmember located in the vertical wall of the gas supply means, the tubularmember being bent in such a way that its ends are directed towards theelectrode surface.
 11. The plasma chemical reactor according to claim 8,wherein the discharge channel includes at least with one vortex chamber.12. The plasma chemical reactor according to claim 11, wherein: thevortex chamber is comprised of a cylindrical groove coaxial with, and oflarger diameter than the diameter of the discharge channel; and the gaschannels are oriented at an angle to the side walls of the vortexchamber such that the vortex flow of the gas is produced.